Net::Http The plain HTTP request was sent to HTTPS port

Question

I'm using the Bluepay API to process payments in my Rails app. The following code is setting up a POST request to the server. RootCA is the path to the SSL certs; locally this is just set to '/'

ua = Net::HTTP.new(SERVER, 443)
ua.use_ssl = true

# Checks presence of CA certificate
if File.directory?(RootCA)
  ua.ca_path = RootCA
  ua.verify_mode = OpenSSL::SSL::VERIFY_PEER
else
  puts "Invalid CA certificates directory. Exiting..."
  exit
end

This works fine locally after calling ua.post(<args>). On Heroku however, the following error is returned:

400 Bad Request

The plain HTTP request was sent to HTTPS port

I have set the SSL certificate path to "/usr/lib/ssl/certs/" as instructed in this Quora answer, but I'm still getting an error.